BQ24072T_1 Datasheet, PDF (22/34 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

bq24072T
bq24075T, bq24079T
SLUS937A – DECEMBER 2009 – REVISED APRIL 2010
www.ti.com
BATTERY DETECTION AND RECHARGE
The bq2407xT automatically detects if a battery is connected or removed. Once a charge cycle is complete, the
battery voltage is monitored. When the battery voltage falls below V_RCH, the battery detection routine is run.
During battery detection, current (I_BAT(DET)) is pulled from the battery for a duration t_DET to see if the voltage on
BAT falls below V_LOWV. If not, charging begins. If it does, then it indicates that the battery is missing or the
protector is open. Next, the precharge current is applied for t_DET to close the protector if possible. If V_BAT < V_RCH,
then the protector is closed and charging is initiated. If V_BAT > V_RCH, then the battery is determined to be missing
and the detection routine continues.
TERMINATION DISABLE (TD Input, bq24072T)
The bq24072T contains a TD input that allows termination to be enabled/ disabled. Connect TD to a logic high to
disable charge termination. When termination is disabled, the device goes through the pre-charge, fast-charge
and CV phases, then remains in the CV phase. During the CV phase, the charger maintains the output voltage at
BAT equal to V_BAT(REG), and charging current does not terminate. The charge current is set by I_CHG or I_INmax,
whichever is less. Battery detection is not performed. The CHG output is high impedance once the current falls
below I_TERM and does not go low until the input power or CE are toggled. When termination is disabled, the
pre-charge and fast-charge safety timers are also disabled.
BATTERY DISCONNECT (SYSOFF Input)
The bq24075T and bq24079T feature a SYSOFF input that allows the user to turn the FET Q2 off and
disconnect the battery from the OUT pin. This is useful for disconnecting the system load from the battery,
factory programming where the battery is not installed or for host side impedance track fuel gauging, such as
bq27500, where the battery open circuit voltage level must be detected before the battery charges or discharges.
The CHG output remains low when SYSOFF is high. Connect SYSOFF to VSS, to turn Q2 on for normal
operation. SYSOFF is internally pulled to VBAT through ~5 MΩ resistor.
DYNAMIC CHARGE TIMERS (TMR Input)
The bq2407xT devices contain internal safety timers for the pre-charge and fast-charge phases to prevent
potential damage to the battery and the system. The timers begin at the start of the respective charge cycles.
The timer values are programmed by connecting a resistor from TMR to VSS. The resistor value is calculated
using the following equation:
t_PRECHG = K_TMR × R_TMR
t_MAXCHG = 10 × K_TMR × R_TMR
Leave TMR unconnected to select the internal default timers. Disable the timers by connecting TMR to VSS.
Note that timers are suspended when the device is in thermal shutdown, and the timers are slowed proportionally
to the charge current when the device enters thermal regulation.
1. During the fast charge phase, several events increase the timer durations.
2. The system load current activates the DPPM loop which reduces the available charging current
3. The input current is reduced because the input voltage has fallen to V_IN-DPM
4. The device has entered thermal regulation because the IC junction temperature has exceeded T_J(REG)
During each of these events, the internal timers are slowed down proportionately to the reduction in charging
current. For example, if the charging current is reduced by half for two minutes, the timer clock is reduced to half
the frequency and the counter counts half as fast resulting in only one minute of "counting" time.
If the precharge timer expires before the battery voltage reaches V_LOWV, the bq2407xT indicates a fault condition.
Additionally, if the battery current does not fall to I_TERM before the fast charge timer expires, a fault is indicated.
The CHG output flashes at approximately 2 Hz to indicate a fault condition. The fault condition is cleared by
toggling CE or the input power, entering/ exiting USB suspend mode, or an OVP event.
STATUS INDICATORS (PGOOD, CHG)
The bq2407xT contains two open-drain outputs that signal its status. The PGOOD output signals when a valid
input source is connected. PGOOD is low when (V_BAT + V_IN(DT)) < V_IN < V_OVP. When the input voltage is outside
of this range, PGOOD is high impedance.
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